Technical Field
The present invention relates to a switching power supply that has a simple configuration and makes it possible to reduce switching loss in a switching element as well as reduce power consumption while in a standby mode.
Background Art
One example of a switching power supply that can provide a rated power capacity on the order of several dozen watts is the flyback power switching circuit illustrated in FIG. 4. This type of switching power supply includes a diode bridge circuit DB that full-wave rectifies AC power supplied from a commercial 100V or 220V AC power source on the input side of the power switching circuit and an input capacitor Cin that smooths the output from the diode bridge circuit DB.
As illustrated in FIG. 4, an AC power input line arranged upstream of the diode bridge circuit DB includes first and second noise filters NF1 and NF2 as well as a capacitor Cx in order to prevent high frequency conductive noise (electromagnetic interference (EMI)) generated during operation of the switching power supply from leaking back to the AC power input line side. Moreover, a resistor Rx is connected in parallel to the capacitor Cx in order to discharge the charge stored in the capacitor Cx when the power source is shut off.
The device main body (the power switching circuit) 1 that forms the main portion of the switching power supply includes a switching element Q that is connected to the diode bridge circuit DB via a primary coil Ta of a transformer T and is switched ON and OFF to control the current that flows through the primary coil Ta. This switching element Q is constituted by a high power capacity MOSFET selected according to the desired power capacity rating for the switching power supply, for example. The device main body 1 further includes a diode D that rectifies an alternating voltage induced in a secondary coil Tb of the transformer T as the switching element Q is switched ON and OFF and an output capacitor Cout that smooths the rectified output from the diode D. Together, the diode D and the output capacitor Cout form a voltage output circuit that generates a prescribed output voltage Vout.
A control circuit 2 integrated as part of a power supply IC switches the switching element Q ON and OFF according to a feedback signal from an output voltage detection circuit 3 that detects the output voltage Vout, for example. The output voltage detection circuit 3 divides and detects the output voltage Vout via voltage-dividing resistors Ra and Rb that are connected in series and includes a shunt regulator SR that calculates the voltage difference between the detected output voltage Vout and a predetermined reference voltage that defines a target output voltage, for example.
Furthermore, the output voltage detection circuit 3 feeds the voltage difference obtained by the shunt regulator SR back into the control circuit 2 as the feedback signal via a photocoupler PC, for example. The control circuit 2 then feedback-controls the pulse width (ON time) of a drive signal that turns the switching element Q ON and OFF according to the received feedback signal, thereby regulating the output voltage Vout to the target output voltage, for example.
The control circuit 2 includes a voltage-controlled oscillator in which the oscillating frequency is controlled by a control voltage. This voltage-controlled oscillator generates a triangle wave signal using the charges and discharges of a built-in capacitor and also generates a rectangular wave signal that is synchronized with the triangle wave signal. The control circuit 2 also includes a pulse-width modulation (PWM) control comparator that compares the voltage of the triangle wave signal generated by the oscillator to the voltage VFB of the feedback signal in order to generate a control signal having a pulse width that defines the ON time Ton of the switching element Q. The control signal output from the comparator is input to a driver circuit arranged on the output side of the control circuit 2. This driver circuit then generates and outputs the drive signal that turns the switching element Q ON and OFF.
This type of output voltage Vout control scheme is widely used in switching power supplies in the 10 to 90 W class in which the output voltage Vout is 12V, 19V, or 32V and is typically known as a secondary-side regulated scheme. Meanwhile, in switching power supplies in the 10W class in which the output voltage Vout is 5V and the output current is less than or equal to 2A, a so-called primary-side regulated scheme (not illustrated in the any of the figures here) in which the output voltage Vout is regulated according to a voltage induced in an auxiliary coil of the transformer T is more commonly used.
The control circuit 2 controls the switching frequency fsw of the switching element Q according to the voltage VFB of the feedback signal, which changes according to the magnitude of the load on the switching power supply. This makes it possible to implement a frequency control scheme that reduces the switching loss in the switching element Q. As is described in detail in Patent Document 1, for example, in this type of frequency control scheme, the switching frequency fsw of the switching element Q is typically decreased in accordance with the voltage VFB of the feedback signal when that voltage VFB becomes less than a prescribed threshold value.
More specifically, as illustrated in FIGS. 5A and 5B, for example, in this type of frequency control scheme the switching frequency fsw is reduced in accordance with decreases in the voltage VFB within a range defined by a maximum switching frequency fsw-max (such as 65 kHz) for when a maximum load is applied and a minimum switching frequency fsw-min (such as 25 kHz) for when a light load is applied. Furthermore, when the load power (the voltage VFB) decreases even further, the switching frequency fsw is reduced to a frequency less than the minimum switching frequency fsw-min such as approximately 0.5 kHz in order to further reduce switching loss in the switching element Q, for example. This type of switching frequency reduction control scheme makes it possible to implement a so-called standby mode in which the resulting reduction in power consumption is maximized. Moreover, this type of frequency reduction control is used widely but exclusively as part of the abovementioned primary-side regulated control schemes.
Furthermore, Patent Document 2 discloses a so-called burst switching control scheme. As illustrated in FIGS. 6A and 6B, for example, in this type of control scheme the additional reduction in the switching frequency fsw while transitioning to standby mode is replaced by an intermittent burst switching drive scheme in which the switching element Q is turned ON and OFF at a prescribed frequency in order to achieve the desired reduction in power consumption in standby mode. This type of burst switching control scheme is widely used in secondary-side regulated switching power supplies.
Moreover, although this is not directly related to the main aspects of the present invention, Patent Document 3 discloses switching a plurality of FETs that are connected in parallel ON in order under prescribed operating conditions and in accordance with load-dependent output currents from a plurality of power supply circuits (more specifically, in accordance with increases in the output currents) in order to balance the load between the power supply circuits. However, in the technology disclosed in Patent Document 3, the plurality of FETs are simply being used as a current output switch. Furthermore, connecting a plurality of switching elements Q that each have a prescribed power capacity together in parallel in order to achieve the desired power capacity rating for the overall switching power supply is a conventionally well-known and widely used technique.